1. Field of the Invention
This invention relates to antenna structures and more particularly to a compact wideband notched radiating element for incorporation into a planar array antenna.
2. Description of the Prior Art
Open ended slot or notched radiating elements are known in the art. Such notched radiating elements are advantageous because they have a relatively wide bandwidth. Notch radiators have other important advantages which are desirable, such as being light in weight and able to be inexpensively manufactured with printed circuit board techniques that are capable of accurate replication from unit to unit.
Often, notch radiating elements are used in wide scanning planar array antennas. Within the array, each notched radiating element is spaced an equal distance apart, with that distance being roughly a half wavelength at the highest operating frequency. This spacing ensures that there is no occurrence of grating lopes that rob the antenna of gain and directionality. Because of this near "square" aperture that each element sees in free space, it becomes necessary to transform typically low impedances (30 to 100 ohms) in microwave power dividers to the quite high impedances (200 to 400 ohms) of near square apertures. Compounding this transform problem are the requirements to make radiating elements less deep and have multiple octave bandwidths.
The bandwidth of a notch radiator is a function of the impedance step and the geometry of the notch. Previously, to get a truly broadband radiating element, one that operates over a wide frequency band, the notched element must be made relatively deep, one to one and a half wavelengths deep, because the impedance ratio between each radiating element feed and free space can be high--nearly 10:1. Therefore, an impedance transformer is needed to lower the impedance of the radiating element.
One way in which the transformer can be incorporated into the radiating element is by using transformers in the form of notches, which is done by making the notch deeper. The transformer may also be accomplished by placing transformer steps on the transmission line feeding the radiating element. The drawback to this approach is that the transformer steps take up a relatively large amount of room in a crowded array environment and we find that the last stage of the transformer to be greater than 200 ohms (for multiple step) and is usually not achievable inside of a microwave package.
Under either transformer approach, the radiating element must be designed to have a large depth so that it may accommodate either the depth of the notch needed for the notch transformer or the transformer steps of the transmission line. The restrictions in the amount of room available in a planar array often limit the possible depth of the radiating element which in turn limits the bandwidth of the antenna.